1. Field of the Invention
The present invention relates to a method of sealing the cells of a ceramic honeycomb body, for trapping diesel particulates, and a cell sealing material.
2. Description of the Related Art
As a filter structure for trapping the particulates in the exhaust gas of an automotive vehicle, for example, a ceramic honeycomb structure 8 as shown in FIGS. 5(a) and 5(b), which comprises a multiplicity of cells 88 defined by a plurality of partitioning walls 81, is available. Alternate ends of a part of the cells 88 are closed by a closing material 830 thereby to make up a plurality of closed portions 83.
In fabricating this ceramic honeycomb structure 8, as disclosed in Japanese Unexamined Patent Publication No. 9-25180, the first step is to mix and knead silicon carbide powder with methylcellulose, making up a binder, water and emulsion stearate, as a lubricant and, using this mixture, fabricate a ceramic honeycomb body with the ends of each cell 88 open. After that, one end of each cell is closed by being filled with a closing material 830.
In closing the cell ends of the ceramic honeycomb body 86, as shown in FIGS. 6(a) and 6(b), an end surface of the ceramic honeycomb body 86 is covered with a wax sheet 91 of a masking material and pressed thereby to pack the wax 90 into the ends of the cells. Then, as shown in FIG. 6(c), the wax 90 to be filled in the cell end portions to be closed is removed manually or by laser thereby to form open cell ends 880.
Then, as shown in FIG. 6(d), with the end surface filled with wax facing downward, the honeycomb structure is dipped in a cell sealing material 60 in slurry form, so that the cell sealing material 60 intrudes into the cell ends 880 lacking the wax 90. The cell sealing material used for this purpose is composed of silicon carbide powder to which methylcellulose, making up a binder, water and trimethyl amine, making up a dispersing agent, are added to form a slurry.
After that, the cell sealing material 60 is dried and baked to remove the wax 90. As a result, alternate ones of the cell ends of the ceramic honeycomb body can be sealed.
In the conventional method of sealing the cells of a honeycomb structure as described above, however, the end portion of the ceramic honeycomb structure is sometimes deformed or dissolved when dipped in the cell sealing material.
The present inventor has vigorously studied the cause of the deformation and dissolution of the dipped portion of the ceramic honeycomb structure, and found the following fact.
Specifically, the methylcellulose contained in the ceramic honeycomb structure is water soluble. Therefore, the ceramic honeycomb structure, once it has absorbed water from the cell sealing material, softens and is deformed. When the structure further absorbs water, the methylcellulose making up the binder is dissolved again and the silicon carbide returns to powder and, being unable to hold the shape of the honeycomb, the end portion of the honeycomb is finally dissolved.
In view of the problems described above, the object of the present invention is to provide a method of sealing the cells of a ceramic honeycomb body and a cell sealing material which can suppress the deformation and dissolution of the portion of a ceramic honeycomb structure dipped in the cell sealing material.
According to a first aspect of the invention, there is provided a method, of sealing the cells of a ceramic honeycomb body, comprising the steps of covering a masking material on the unsealed end surface of the ceramic honeycomb body and dipping a predetermined end surface of the honeycomb structure in a cell sealing material thereby to seal the predetermined end surface of the ceramic honeycomb body,
wherein the cell sealing material includes ceramic particles and an assistant for fluidizing the ceramic particles, the assistant having the property of not substantially redissolving the binder contained in the ceramic honeycomb body.
What is most notable of this invention is that an assistant having the property of not dissolving the binder contained in the ceramic honeycomb body is used instead of the water thus far contained in the conventional cell sealing material.
In this aspect of the invention, the assistant is used to give fluidity to the ceramic particles and the binder. This assistant, unlike water, has the property of not dissolving the binder contained in the ceramic honeycomb body. When the ceramic honeycomb body is dipped in the cell sealing material, therefore, the dipped portion is not dissolved in the cell sealing material. As a result, the portion of the ceramic honeycomb body dipped in the cell sealing material can be prevented from being deformed and dissolved.
According to a second aspect of the invention, there is provided a method of sealing the cells of a ceramic honeycomb body in which the assistant is preferably made of at least selected one of a petroleum hydrocarbon, mineral oil, animal or vegetable oil, a higher alcohol and a synthetic oil. In this way, the deformation or dissolution of the dipped portion of the ceramic honeycomb body can be effectively suppressed. These assistants preferably have a high resin solubility, i.e. a low compatibility with water and are recommendably AF Solvent (trade name), for example.
The amount of the assistant contained in the cell sealing material is appropriately selected to make sure that the cell sealing material retains fluidity and the assistant is adsorbed to the wall surface while being dipped in the ceramic honeycomb body so that the cell sealing material is concentrated while being provisionally solidified in the cells of the ceramic honeycomb body.
In the case where AF Solvent (trade name) is used as an assistant, for example, the amount of the assistant contained in the cell sealing material is preferably 50 to 70% by weight. In the case where the content of the assistant is less than 50% by weight, the fluidity of the cell sealing material is so low that the cell end surface may be difficult to seal. In the case where the content of the assistant is more than 70% by weight, on the other hand, the cells may be sealed unevenly.
The ceramic particles contained in the cell sealing material are preferably of the type identical or similar to the ceramic honeycomb body in property. This is to prevent cracking by securing an approximate burning shrinkage ratio and thermal expansion coefficient.
The ceramic particles used for this purpose are composed of a cordierite material, a mullite material, alumina, silicon carbide or silicon nitride.
The cell sealing material may contain a binder. The binder functions to bond the ceramic particles to each other. The binders usable for this purpose include celluloses such as methylcellulose and ethylcellulose, acryl binder and polyvinyl alcohol. Depending on the combination of the material of the ceramic particles and the assistant in the cell sealing material, the binder may be done without. The binder may not be used, for example, in the case where the ceramic particles are composed of a cordierite material and the assistant is AF Solvent.
In the case where an excessive amount of cell sealing material is put in a vessel and the end surface of the ceramic honeycomb body is dipped in the cell sealing material, the cells are not easily sealed by the cell sealing material. The reason is that the ceramic honeycomb body absorbs the assistant not only from the sealed portions but also from the cell sealing material in the vessel, with the result that before the provisional solidification, the assistant absorbed into the ceramic honeycomb body is saturated. Thus, the provisional solidification of the end portion of the ceramic honeycomb body becomes insufficient, and the cell sealing material falls off from the end surface of the ceramic honeycomb body when the latter is taken out of the cell sealing material.
According to a third aspect of the invention, there is provided a method of sealing the cells of the honeycomb body in which, preferably, the cell sealing material is put in a vessel of a sufficient size to accommodate the end surface of the ceramic honeycomb body and the cell sealing material thus charged is of an amount required for one dipping treatment.
The cell sealing material required for one cell sealing treatment is put in the vessel. When the ceramic honeycomb body is mounted in this vessel, the cell sealing material intrudes into the inner wall of the through holes of the mold. The assistant contained in the cell sealing material is absorbed into the ceramic honeycomb body, and the viscosity of the cell sealing material is gradually increased. Before long, the cell sealing material that has intruded into the end portion of the ceramic honeycomb body is provisionally solidified and hardened to such an extent that it will not come off. This method, therefore, can positively seal the end surface of the ceramic honeycomb body.
The amount of the cell sealing material required for one dipping treatment is defined, for example, as an amount larger than the total volume of the sealed portion of the one end surface but smaller than the total volume of the sealed portion of the two end surfaces of the ceramic honeycomb body.
The surface area of the vessel for containing the cell sealing material is preferably as small as possible to the extent that the ceramic honeycomb body can be accommodated and recovered smoothly. This is in order to minimize the amount of the assistant used for one dip session and secure provisional solidification.
In producing the ceramic honeycomb body, ceramic particles are mixed with a binder, water and, when necessary, a lubricant, and after being kneaded, formed into a honeycomb structure using a mold and solidified by being dried.
According to a fourth aspect of the invention, there is provided a cell sealing material for sealing the cells in the end surface of a ceramic honeycomb body,
wherein the cell sealing material is configured of ceramic particles and an assistant for fluidizing the ceramic particles, the assistant having such a property as not to substantially redissolve the binder contained in the ceramic honeycomb body.
The cell sealing material according to this aspect of the invention contains, in place of water, an assistant having such a property as not to substantially redissolve the binder contained in the ceramic honeycomb body. The portion of the ceramic honeycomb body dipped in the cell sealing material, therefore, is prevented from being deformed and dissolved.